An intravascular diagnostic apparatus utilizing an optical technology has greatly progressed in recent years. As a representative example of apparatuses utilizing the optical technology, an optical coherence tomography (OCT) diagnostic apparatus is exemplified. By using this apparatus, it has become possible to observe the property and state (for example, cross section of blood vessel and its inner surface) of a blood vessel and in addition, visualize an image to be observed in three dimensions and quantify the property and state of the blood vessel. The optical coherence tomography diagnostic apparatus depicts the image of the inner surface of the blood vessel, based on light reflected from in-vivo tissues by inserting an optical fiber having a probe incorporating an optical lens and an optical mirror mounted at a front end thereof into the blood vessel and by emitting light into the blood vessel with an optical mirror disposed at the front end of the optical fiber radially scanning the inner surface of the blood vessel. In another known image diagnostic apparatus, an optical frequency domain imaging (OFDI) method called a next-generation OCT is used, as disclosed in Japanese Patent Application Laid-Open Publication No. 2011-206375 (patent document 1 proposed by the present applicant).
When the intravascular image diagnosis is conducted, a catheter for use in the intravascular image diagnosis is delivered to a portion to be observed through a guide wire, In performing the intravascular image diagnosis, reflection of light and ultrasonic waves may occur owing to blood containing blood cell components such as red blood cells, which hinders the formation of tomographic and inner surface images of a blood vessel to be diagnosed with high accuracy. Thus in performing the intravascular image diagnosis, it is necessary to remove the blood cell components from the blood vessel to be diagnosed.
At a clinical site, image diagnosis is carried out after a state in which the blood cell components are removed from the blood vessel is temporarily produced by injecting a liquid such as a contrast agent having a high viscosity or saline into the blood vessel. All operation of discharging the liquid to the blood vessel is called a flush operation. The liquid discharged in the flush operation is so-called a flush solution.